No Redshift Evolution in the Broad-line-region Metallicity up to z = 7.54: Deep Near-infrared Spectroscopy of ULAS J1342+0928

Abstract

We present deep (9 hr) Gemini-N/Gemini Near-InfraRed Spectrograph near-infrared spectroscopic observations of ULAS J1342+0928, a luminous quasar at z  = 7.54. Various broad emission lines were detected, as well as the underlying continuum and iron forests over the rest-frame wavelength 970–2930 Å. There is a clear trend that higher-ionization emission lines show larger blueshifts with C iv exhibiting km s −1 blueshift with respect to the systematic redshift from the far-infrared [C ii ] emission line. Those high-ionization lines have wide profiles with FWHM more than 10,000 km s −1 . A modest blueshift of km s −1 is also seen in Mg ii , the lowest-ionization line identified in the spectrum. The updated Mg ii -based black hole mass of and the Eddington ratio of confirm that ULAS J1342+0928 is powered by a massive and actively accreting black hole. There is no significant difference in the emission-line ratios such as Si iv /C iv and Al iii /C iv when compared to lower-redshift quasars in a similar luminosity range, which suggests early metal pollution of the broad-line-region clouds. This trend also holds for the Fe ii /Mg ii line ratio, known as a cosmic clock that traces the iron enrichment in the early universe. Different iron templates and continuum fitting ranges were used to explore how the Fe ii /Mg ii measurement changes as a function of spectral modeling. Quasars at even higher redshift or at fainter luminosity range ( erg s −1 ) are needed to probe the sites of early metal enrichment and a corresponding change in the Fe ii /Mg ii ratio.